In prior art transmission systems using the correlation code, a specific coding rule is introduced that governs a plurality of adjoining input data to narrow the band width of an output signal.
For example, in a system with a bit rate of f.sub.0, a secondary bipolar (modified duobinary) code, disclosed in a literature entitled "Advance in PCM Communication" (Sanpo) by Inose and Miyagawa at page 89, has a main lobe of less than f.sub.0 /2 and its band is one fourth that of the ordinary non-return-to-zero (NRZ) code.
In a transmission system using the correlation code, the output at a certain time is determined by a combination of a present input as well as past inputs, so that it is difficult to fix a frame synchronization pattern. For this reason, a new frame synchronization method was proposed in the Japanese Patent Laid-Open No. 119947/1984 "Partial Response Transmission Method." This method establishes the frame synchronization by deliberately inserting a code violation pattern although the frame synchronization pattern is not fixed.
With the above prior art (Japanese Patent Laid-Open No. 119947/1984), while the frame synchronization becomes possible, the frame synchronization pattern varies from one frame to another because the pattern at the transmitter depends on the past inputs. Therefore, no fixed frame synchronization patterns can be observed on the transmission line. Another problem is that when a precoder, described at page 83 of the above reference "Advance in PCM Communication," is used, not only does the dependence of the output of the coder on all the past data make it impossible to fix the frame synchronization pattern, but also bit errors that occurred in the precoder circuit affect the data at succeeding stages. Therefore, it is necessary to suppress this error propagation.